Soybean rust findings cause concern

Soybean rust was found at the end of February at three sites in Montgomery County, Ala., creating concern for over-wintering of disease spores farther north than central Florida.

The three sites in Alabama were found on kudzu in areas protected by old buildings. Finding live spores in Montgomery County is significant, for two primary reasons: Montgomery is in the center of Alabama, roughly 140 miles north of the Gulf of Mexico and the area recently recorded low temperatures in the mid 20s F, which caused die back of most annual plants, but did not destroy these host plants.

Previously, rust was found growing in sites in Mobile and Baldwin counties in Alabama, though these counties are located in the southernmost part of the state, along the Gulf of Mexico.

At a site in Brooks County, Ga., live rust inoculum was found growing on kudzu, where rust inoculum had been destroyed earlier in the year. This site was likewise subjected to temperatures in the 28 degree F range during the recent Arctic cold front that crossed the area. Brooks County is located in south Georgia, along the Florida line.

Marvin Miller, a plant pathologist at Texas A&M University's Weslaco Research Center reported finding rust inoculum in a field at the research station located near the border with Mexico. The live rust found in late February was not present in the field, when it was scouted in January.

Rust was also found in Tamaulipas, Mexico, across the border from Weslaco. Miller speculates this is the most likely source of spread. He points out that drought conditions in south Texas may keep spores in the atmosphere and move them around even more when rains come.

Finding rust in Texas is a concern to Greg Shaner, a plant pathologist at Purdue University. While the Texas report may mean a greater risk to growers in the Midwest, Shaner contends rust spores may not move as quickly as he once believed.

“We think ultra violet light in the atmosphere may break down or even destroy the viability of inoculum.” He points out that spores found in 2005 in Indiana, Illinois and Minnesota did not cause disease. Viable spores, he says, may only spread by short hops from field to field.

In addition to the sites in Texas, Georgia and Alabama, soybean rust has been found in 11 counties in Florida. With the traditional last freeze date in northwest Florida coming around March 9, there is some concern that the area may not receive another Arctic front capable of producing temperatures low enough to kill host plants.